Re: Clean Object Class Design -- Circle/Ellipse

From: James A. Robertson <jarober_at_mail.com>
Date: Sun, 07 Oct 2001 04:42:47 GMT
Message-ID: <3BBFDD8E.4020802@mail.com>

Using a real example, show me how any of your points matter in Smalltalk.

Bob Badour wrote:

> brangdon_at_cix.co.uk (Dave Harris) wrote in message news:<memo.20011001000009.35593A_at_brangdon.madasafish.com>...
>

>>bbadour_at_golden.net (Bob Badour) wrote (abridged):
>>
>>>Ask him to point to any Smalltalk language definition that defines
>>>"instance" as anything other than a variable which one can reference.
>>>
>>The 1997 draft of the ANSI Smalltalk standard (which is the version of the 
>>standard I have to hand), says:
>>
>>    A /variable/ is a computational entity that stores a single
>>    reference (the /value/ of the variable) to an object.
>>
>>    [...] An /object/ is a computational entity which is capable of
>>    responding to a well defined set of messages. An object may
>>    also encapsulate some (possibly mutable) state.
>>
>>This is *not* saying an object is a variable.
>>

>
> "reference to an object" implies that "object" is variable. Values are
> self-identifying and one cannot reference a value. One can reference
> storage or memory that contains some representation of a value, but
> storage and memory are variable. That a variable is constrained to a
> single value for some period of time does not change its nature.
>
> That's not to say that no such thing as an object value exists. Object
> value is an important concept just as object variable is an important
> concept.
>
> If you had included a little more from the standard where it defines
> /message send/ and explains that "Each argument is a reference to an
> object", you would see that Smalltalk requires a variable in order to
> send a message. In other words, Smalltalk only pretends to have values
> and requires one to store a representation of a value in a variable
> prior to doing anything.
>
>
>

>>(The ANSI standard does not define "instance". It uses "object" instead. 
>>Later it says that when an object does have state, it represents it with 
>>special kind of variables called "instance variables". In other words, an 
>>object is not a variable but an object can contain variables.)
>>

>
> How does one change part of something that cannot change?
>
>
>

>>>An instance or variable can have the value 5. The value, however, is 
>>>not an instance.
>>>
>>So sorry, but in Smalltalk the value 5 is an instance of the class 
>>SmallInteger. 
>>

>
> Untrue. An instance of the class SmallInteger can contain a
> representation of the value 5, but the value 5 is not an instance of
> anything. I realize that Smalltalk confuses things by requiring
> variables for messaging and then pretending that variables are values.
>
>
>

>>> One cannot reference a value. Values are self-identifying.
>>>
>>Indeed 5 is self-identifying. We can send a message to the value 5 whether 
>>or not we have a variable referring to it. 
>>

>
> Yes, I agree that one can have operations on values. These are, of
> course, different from update operations on variables. Have you not
> seen me state that previously? Of course, the Smalltalk standard
> requires all implementations to operate only on variables.
>
>
>

>>>According to the definition of the Smalltalk language an instance is a
>>>variable that one can reference.
>>>
>>Again, so sorry, but my reading of the ANSI definition of Smalltalk says 
>>that an instance (or object) is something which responds to messages. 
>>

>
> My reading of the same says that one can reference an instance (or
> object), which means it must have some physical existence. Variables
> have physical existence where each stores some representation of a
> value; values have no physical existence. Values exist regardless of
> physical representation.
>
>
>

>>Perhaps you are talking about some other language, also called Smalltalk? 
>>

>
> No. I just read with a little extra care and a little better
> understanding of the concepts involved.
>
>
>

>>How confusing! Can you point me to a definition of your language? 
>>

>
> The one above definition will suffice. You should also read section
> 3.3.2 very carefully.
>
>
>

>>Definitions of ANSI Smalltalk are available from:
>>  http://minnow.cc.gatech.edu/squeak/172
>>
>>  Dave Harris, Nottingham, UK | "Weave a circle round him thrice,
>>      brangdon_at_cix.co.uk      |   And close your eyes with holy dread,
>>                              |  For he on honey dew hath fed
>> http://www.bhresearch.co.uk/ |   And drunk the milk of Paradise."
>>

Received on Sat Oct 06 2001 - 23:42:47 CDT